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I have a four and half year old car. I have recently noticed that there is white powder forming to the battery terminals. The car still has the original battery and probably should be replaced sometime in the near future.

What is this substance?

Is it harmful?

Should I attempt to remove it or should I just leave it be there?

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    Your on a tear! Merry Christmas. Love your questions and if I didn't say it before, welcome to the site. Cheers – DucatiKiller Dec 24 '15 at 20:03
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    Thanks! I figured out that since the site is in beta, I could as well provide some useful material that could help this site become permanent. I think there is a genuine need for a motor vehicle maintenance Q&A site. Merry Christmas to you too! – juhist Dec 24 '15 at 20:10
  • It is dangerous, clean it off and smear some dielectric grease on those battery terminals to help prevent it from coming back. – MasterHD Dec 25 '15 at 15:03
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    Update: I actually rembembered it incorrectly. I found that the white stuff is on the battery hold down bracket and there is a Q&A for it already. The terminals are clean. But I won't remove this question because somebody may have the white stuff on the battery terminals, and may benefit from the excellent answers. – juhist Dec 25 '15 at 16:51
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Background

Yes it's harmful.

It is lead sulphate.

It is toxic to ingest and breath.

It is especially toxic to children, as you would imagine regular lead to be.

It's not miscible in water but baking soda (sodium bicarbonate) neutralizes that acidic portion of the compound. The remnant compound of that reaction would be H2O and some CO.

Action

Use gloves. Lead salts are quite poisonous. You should also wear an air filtering mask and eye protection when cleaning it.

You can put baking soda in some water and mix it. Pour it on the battery terminal and use a brush to clean it off. The water and baking soda will prevent you from breathing in any of the lead sulfate dust. You will see a reaction and it will foam, that will be mostly water and CO with ions of a few other chemicals like lead and sulpher, there is a type of salt that breaks off too.

If you see any brown in the foam that is probably iron oxide getting broken down from an older component of the cable connector, if you have an older vehicle. Modern clamps avoid the use of ferrous metals due to oxidation (rust)

Use distilled water to pour on the foamy mess to wash it off. Ensure you DO NOT get any of the remnants from the reaction on the paint of the vehicle. It will damage it. Ensure you wash the remnants completely out of the vehicle, battery tray, devices below the tray like suspension etc.

Hope this helps. Cheers.

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    To add one little bit, ensure all of the foamy mess is completely washed away, not only from the battery, but from the battery tray and the inside of the engine bay. This stuff will cause paint to go bye-bye and then rust will ensue over time. Get it clean or suffer the consequences. – Pᴀᴜʟsᴛᴇʀ2 Dec 24 '15 at 20:05
  • @Paulster2 Good point, I'll add that bit in. – DucatiKiller Dec 24 '15 at 20:05
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This is anhydrous sulfuric acid that has leaked from around the terminal posts. It will rather quickly corrode the battery terminal connectors. Rinse it off with water first, to get most of it off, flush with lots of water. Then put a mixture of baking soda and water in the areas affected, then rinse that off.

To keep it from returning treat the terminals with an acid neutralizing substance. After trying most everything we have used NOCO brand products with outstanding success.

Some say Vaseline grease works is best, we have not found to work as well.

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